[chronik] fix blockchain.block.headers electrum endpoint

Summary:
We accidentaly returned an error when more than 500 headers were requested. The limit relevant to the Electrum protocol is 2016, and to match Fulcrum's behavior we don't return an error if we exceed it, we just cap the number of headers to 2016.

Depends on D18563

Test Plan:
ninja check-functional

Test it locally on mainnet:

./src/bitcoind  -chronik  -chronikelectrumbind="127.0.0.1:50001:t"

echo '{"jsonrpc": "2.0", "method": "blockchain.block.headers", "params": [800000, 2016], "id": "test"}' | nc 127.0.0.1 50001

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18562